Sis a subset within a universal set, u. if s = {x, y, 4, 9, which could describe u? u = {keys on a keyboard} u = {letters} u = {numbers} u = {punctuation marks}

S is a subset of U, so all of the elements of S must be also elements of U

If U was {letters}, 4, 9, ? would not belong there
If U was {numbers }, x, y, ? would not belong to U
If U was {punctuation marks }, 4, 9, x, y would not be in U

all the elements of S are in U={ keys on a keyboard}

  U={ keys on a keyboard}